Clues to mathemagics
By ilunice
@ilunice (947)
Netherlands
January 10, 2007 12:16pm CST
Here we go:
The magic number that will divide any number whose sum of digits is equal to itself is 9. Example: 90/9=10, 9+0=9, 99/9=11; 9+9=18 and 1+8=9; 36/9=4 and 3+6=9 QED
The number that denotes the union between male and female is 5. 5 is the first number on the numberline that represents addition of the first odd number (3) and the first even number (2). So if 3 = male and 2 = female, then, 5 = unity of relationship. QED
Any number divided by itself is unity. Exeption are: 0/0 = indeterminate; infinity /infinity =infinity.
5 responses
@WebGal (48)
• United States
10 Jan 07
Actually, the first odd number is 1, and infinity/infinity is indeterminate, just like 0/0.
A number is divisible by 2 if its last digit is even.
A number is divisible by 3 if the sum of its digits is divisible by 3.
A number is divisible by 4 if either: the next-to-last digit is even and the last digit is 0,4 or 8, or the next-to-last digit is odd and the last digit is 2 or 6
A number is divisible by 5 if the last digit is 0 or 5.
A number is divisible by 6 if it is divisible by both 2 and 3.
Here's a little-known test for divisibility by 7; it's more complicated than the others but with practice you can still do it in your head. Split off the last digit from a number, double the last digit and subtract that from the upper digits. For example, take '371'. Split off the last digit, '1', leaving '37' Double the '1' to give '2'. Subtract: 37 - 2 = 35, which is divisible by 7.
You can repeat the process as needed; take a larger number like 75061:
7506 - 2*1 = 504
750 - 2*4 = 742
74 - 2*2 = 70 -- divisible by 7, so 75061 is also divisible by 7.
If 'double the last digit' is greater than the remaining digits, you can subtract the other way. Take '98' for instance; the rule says to compute '9 - 2*8' (or '9 - 16') but you can reverse the numbers to give '16 - 9', that is, '7'. So 98 is divisible by 7.
@gigarange (1165)
• United States
10 Jan 07
Hey That is so cool. That splitting the digit thing. Thanks for sharing that, I diddn't know that. Just now. I surely would try that in some other numbers maybe then I can master it and would be easier for me the next time.
@josephperera (2906)
• Sri Lanka
10 Jan 07
A two digit number can be multipled by 11 very easyily. 23 into 11 is 253. That is 2(2+5)3. 54 into 11 is = 5(5+4)4 = 594. You simply add the two numbers and put it in the middle. The what about 97 into 11. It is 9(9+7)7 = 9(16)7. Since the two number add up to a number over 10 keep the right hand side number in the middle and add the left hand side number to the left digit. So it will be 1067. So 84 into 11 will be = 8(8+4)4 = 8(12)4 = (8+1)(2)4 = 924.
@gigarange (1165)
• United States
10 Jan 07
This is a nice one. Thanks for sharing this information. I would try it in any other number and maybe I can memorize it then.
